1. Field of the Invention
The present invention generally relates to fiber optic bundle-based illumination, and, more particularly, to a system and method for fiber optic bundle-based illumination for an imaging system.
2. Description of the Related Art
Light can be delivered from a remote source to a destination in several ways. One way in which this may be achieved is by transmitting the light in free space. Another way is to transmit the light through optical fibers.
When used in microscopy, optical fibers allow an illumination light source, photodetectors, or both, to be located remotely from the microscope. For example, Publication WO95/11461 and Hooper et al., “Developments and Applications of Photon Imaging for Biomolecular Screening,” The Society For Biomolecular Screening (1995), the disclosures of which are incorporated by reference in their entireties, disclose the use of a fiber optic taper to transmit from a sample to a CCD detector.
There are two basic categories of optical fibers: single mode optical fibers that are specific to a wavelength and can guide only a single spatial mode, and multimode optical fibers that can guide more than one wavelength and multiple spatial modes.
Single mode optical fibers are well-suited for confocal illumination delivery because the single spatial mode can be focused to a near diffraction-limited spot in the specimen plane of the microscope. This allows high resolution imaging. Multimode optical fibers, however, are commonly used for both delivery of light and fluorescence collection because they possess larger core diameters and usually have greater numerical aperture values. Optical fibers and their uses in microscopy are discussed in detail in Flushberg et al., “Fiber-optic fluorescence imaging,” Nature Methods 2:12 941-50 (December 2005) (“Flushberg”), and Helmchen, “Miniaturization of fluorescence microscopes using fibre optics,” Exp. Physiol 87.6 737-45 (2002), the disclosures of which are incorporated by reference in their entireties.
Optical fibers may also be grouped together in what are often referred to as fiber optic bundles. Fiber optic bundles may consist of a plurality of individual single mode or multimode optical fibers and are commonly used for fluorescent illumination and scanning confocal imaging. See Flushberg at 944. For example, when used in a line confocal system, fiber optic bundles may be used to transmit light from the light source to the sample. As illustrated in FIG. 2b of Flushberg, light from a light source is reflected by an one-dimensional scanner and then passed through a cylindrical lens. The cylindrical lens focuses the light into one axis, such as the horizontal or vertical axis. The focused light then enters a fiber bundle long the selected axis and is delivered to the sample.
Although effective, the approach illustrated in Flushberg has several drawbacks. For example, because cladding around each optical fiber creates a spacing between the light-carrying portions of the optical fibers, the light output from the bundle will not be uniform. This reduces spatial resolution of both the illumination and collection light. This pixilation can reduce the lateral optical resolution in the specimen plane by about twice the average core-to-core distance between fibers divided by the optical magnification of the imaging lens. See Flushberg at 944.
In fluorescence microscopy it is common to use more than one fluorescent dye in a sample. Accordingly, it is often necessary to use multiple sources of excitation light for each fluorescent dye. To accomplish this, optical fibers are commonly used to deliver the different light from each light source to the sample. In practice, when conventional fiber optic beam combiners are used, multiple single mode optical fibers may be used to deliver the light from the light source to a single output optical fiber. However, this approach is inefficient because of losses associated with the optical coupling of the input optical fibers to the single output optical fiber. In addition, the single output optical fiber is not optimized for transmission of the typical wavelengths required for fluorescent illumination, i.e., from the near IR to the near UV.